The present invention relates to friction coupling devices and fluid coupling devices, such as friction clutch assemblies and viscous drives; the fluid coupling devices being of the type that include both a fluid operating chamber and a fluid reservoir chamber, and valving to control the quantity of fluid in the operating chamber.
Although the present invention may be used advantageously in various configurations and applications, it is especially advantageous in a coupling device of the type used to drive a radiator cooling fan of an internal combustion engine for a over the road truck, such as a class 8 truck, and will be described in connection therewith.
Friction coupling devices and fluid coupling devices that drive radiator cooling fans are generally of two types, dry friction clutch assemblies and viscous drives, respectively.
Dry friction clutch assemblies tend to have two operating conditions “ON and OFF” referring to when a friction clutch is either fully engaged or fully disengaged. When a friction clutch assembly is providing cooling the clutch is fully engaged and not slipping. When the friction clutch assembly is not providing cooling the assembly is fully disengaged and slip speed is at a maximum between a clutch plate and an engagement surface.
The dry friction clutch assemblies generally have low thermal capacity, since they typically do not incorporate fluid flow cooling mechanisms. Thus, the clutch assemblies have minimal cooling capability and are unable to cycle repeat in short durations of time. Also, because of low thermal capacity, the clutch assemblies are also limited in torsional capacity, such that they are incapable of engaging at high engine revolutions per minute (rpm) or high engine speeds. The thermal energy that is generated during engagement at high engine rpm speeds can “burn up” or cause the clutch assembly to become inoperative.
Viscous drives, on the other hand, have become popular due to their ability to cycle repeat, engage at higher engine speeds, and have varying degrees of engagement. Viscous drives have an operating range of engagement and are generally less engaged at higher engine speeds and generally more engaged at lower engine speeds. Viscous drives are never fully engaged for internal viscous shear purposes.
Unfortunately, viscous drives are also thermally and torsionally limited. Viscous drives are always slipping to some degree causing them to be incapable of turning at fully engaged peak operating speeds or at higher speeds than originally designed. Since viscous drives are continuously slipping, they are continuously generating heat, unlike friction clutch assemblies. Viscous drives are further limited in that the more engine cooling needed the larger and more costly the viscous drive and cooling fan that is required. Thus, for increased engine cooling requirements viscous drives can become impractical in size and cost.
Due to increased engine cooling requirements, a current desire exists for a fan drive system that is capable of not only providing an increased amount of cooling over traditional fan drive systems but also that it have the associated advantages of a friction clutch assembly and of a viscous drive, as stated above, without the associated disadvantages. It is also desirable that the fan drive system be practical and reasonable in size and cost, so as to be approximately similar to and preferably not to exceed that of traditional fan drive systems.